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Crystal structure of a new (21)-clinopyribole synthesized at high temperature and pressure

Hexiong Yang^*, Jürgen Konzett and Charles T. Prewitt

Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, D.C. 20015-1305, U.S.A.

A (21)-clinopyribole with the composition K_1.10Na_2.32Ca_1.52Mg_5.85Al_1.23Si_12.04O₃₄(OH)₂ has been synthesized at 10 GPa and 1250 °C in a multi-anvil apparatus. The unit-cell parameters are a = 9.8390(9), b = 26.6471(6), c = 5.2665(5) Å, ß = 106.25(5)°, and V = 1325.6(4) ų. The structure (space group A2/m) consists of an alternating arrangement of single- and double-chain silicate slabs along the b axis, with a + + + + configuration. This phase possesses all the features predicted by Veblen and Burnham (1978b) for a mixed-chain silicate intermediate between pyroxenes and amphiboles. The single-chain portion of the structure corresponds to a clinopyroxene with the omphacite composition Di₅₅Jd₄₅, whereas the double chain portion is essentially a potassic richterite. The MS2 cation in the single-chain portion occupies a coordination environment that is similar to the M4 site in richterite, whereas the MD4 cation coordination in the double-chain portion is comparable to the M2 site in C2/c omphacite. The observed unit-cell volume is 1.5% smaller than the equivalent mixture of Di₅₅Jd₄₅ + richterite, accounting, in part, for its high-pressure stability relative to its pyroxene and amphibole components.

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